The present invention relates more particulary to tachometer devices of a type known per se comprising a sensor which generates an electrical signal at a frequency proportional to the speed to be detected, an operating drive member, and electrical connection means interconnecting the sensor to the operating drive member.
A conventional tachometer device of this type is shown diagrammatically in accompanying FIG. 1.
As shown in FIG. 1, the sensor 10 may comprise a phonic wheel 11 comprising uniformly spaced teeth 12 around its periphery and associated with a transducer 13 such as a Hall effect transducer, suitable for detecting the passage of the teeth 12 and for generating electrical pulses at its detection rate.
The operating drive member 20 may comprise, as shown in FIG. 1, a polyphase stepper motor element 21 and a system 26 for measuring eddy currents. The motor element 21 comprises stator windings 22, 23, and 24, and a rotor constituted by a main magnet 25. The measuring system 26 comprises an auxiliary magnet 27 coupled to the main magnet 25 in order to rotate therewith, and a bell 28.
The bell is made of an electrically conductive material. It fits over the auxiliary magnet 27 and is guided to pivot coaxially therewith. The main magnet 25 is rotated by the rotating field generated by the windings 22, 23, and 24. The auxiliary magnet 27 drives the bell by virtue of the eddy currents generated therein. A spiral spring 29 fixed to the shaft of the bell 28 limits rotation thereof.
The angle through which the bell 28 pivots relative to a neutral or rest position is directly related to the frequency of the pulses from the sensor 10. The equilibrium position of the bell 28 is determined by the driving couple applied to the bell 28 by the auxiliary magnet 27 being equal to the opposing couple applied to the bell 28 by the spring 29. The shaft of the bell 28 may be fitted with a pointer whose position is directly representative of the speed to be detected of the phonic wheel 11.
The electrical link means interconnecting the sensor 10 and the operating drive member 20 are referenced 30 in FIG. 1.
A frequency divider 40 is preferably interposed between the sensor 10 and the operating drive member 20 in order to apply the pulses generated by the sensor 10 in succession to the various windings 22, 23, and 24.
The Applicants have observed that although tachometer devices of the above type have given good service, they suffer from the following major drawback. Given the inertia of the operating drive member, it remains stationary and does not start moving when the phonic wheel 11 accelerates too violently or when the transducer 13 is suffering from intermittent faults, while the phonic wheel continues to rotate rapidly.
Further, continuing to feed power to the windings of the motor element 21 in the event of a breakdown or when the phonic wheel stops, gives rise to unacceptable heating in the device.
The present invention seeks to eliminate these drawbacks.